Pulsed magnetron instability detector



July 14, 1959 Filed May 28, 1954 2 Sheets-Sheet 1 G. l. KLEIN pm ssn MAGNETRON INSTABILITY DETECTOR July `14, 1959 Filed nay 28, 1954 2 sheets-sheet 2 INVENTOR Gee/:L0 /E/N fla .4. lik/Ligny ATTORN YS:

United States Pate PULSED yMAGNETRON INSTABILITY DETECTOR Gerald I. Klein, New York,- N.Y., assignor to the United States of America as represented by the Secretary of the Navy Application May 28, 1954, Serial No. 433,309

7 Claims. (Cl. 331-87) (Granted under Title 35, U.S. Code (1952), sec. 266) The invention described herein. may -be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment ofy any royalties thereon or therefor.

This invention concerns a pulsed magnetron instability detector circuit and moreparticularly, a pulsed magnetron instability detector circuitv for providing quantitative information on the useful energy content in radio frequency pulses generated rby a pulsed microwave magnetron tube or its equivalent.

Pulsed microwave magnetrons hereinafter referred to |as magnetrons, are subject to many forms of malfunction or instability that all result in the loss of useful radio frequency output power. The existence of instability in a magnetron, the prevalence of a particular form of instability, and the percentage of applied voltage pulses; during which the prevalent form ofinstability is manifested isv a complicated function of the electrical characteristics of the magnetron, the electrical conditions to which it is subjected, and the type of circuitry employed for pulse modulation of the magnetron. All forms of magnetron instability are characterized by the loss of radio frequency output power withinthe desired frequency band. Information on power loss is of prime importance in the quest for improved operating results.

The types :of instability that are encountered in the operation of pulsed magnetrons include arcing, moding, and rnisiiring. Arcing is characterized by a voltage breakdown from anode to cathode due to the formation of a conducting path of ionized gas. Moding is characterized by oscillation -in the magnetron at an undesired frequency produced by a spurious electron space charge configuration that is sustained by circuit operating conditions. Since this usually results in an electromagnetic field pattern in the magnetron resonant circuits that is quite different than that for which the magnetron output coupling section is designed, only a very small fraction of the microwave power `generated in the magnetron is transmitted to the transmission line. Misi'iring is characterized by the absence of oscillation in the magnetron; it results when the voltage-current characteristic of the magnetron and the voltage-current characteristic yof the magnetron driving circuit do not coincide at any point to sustain oscillations. No anode current flows in the magnetron when the magnetron misires.

Techniques available in the present state of the art for studying instability in magnetrons involve the use of instruments that provide only qualitative information. The interpretation of` the qualitative information is dependent upon the experience and judgment of the operator. For example, one such technique entails visual counting of llicks on a cathode ray tube screen when evaluating a pulsed magnetron for instability due to arcing. At best, both the technique and the results obtained are crude. There is no technique or detector available in the present state of the art that can provide quantitative information on magnetron performances. Specifically, there is no method or apparatus that provides quantitative informav2,895,107 Patented July 14, 1959 tion on instability in magnetrons by analyzing a sample of thel radio frequency output pulses from a magnetron to evaluate useful energy content.

This invention marks a departure from the prior art in that it provides a method and apparatus for providing quantitative data that directly evaluate the stability of magnetrons. This invention samples the radio frequency output pulses of a magnetron, properly yanalyzes each pulse, and classifies each pulse according to how much of its energy content is in the desired frequency band. Each radio frequency pulse generated by the magnetron under test is processed through a narrow bandpass filter.- The filter blocks all radio frequency energy not generated in the desired voltage mode of oscillation within the magnetron. Each pulse or any fraction of a pulse passed by the filter is detected and its envelope is integrated to measure the energy content.V A countery registers the number pulse periods during which a magnetron does not generate as much or more than a particular quantity of radio frequency energy n;- the desired frequency band. 'llhe instability detector circuit may be adjusted to vary the level of the aforesaid particular quantity of radio frequency energy.

An object of this invention is to provide a pulsed magnetron instability detector circuit.

A further object is to provide a pulsed magnetron instability detector circuit for alfording accurate quantitative information on instability in magnetrons.

A further object is to provide a pulsed magnetron instability detector circuit for 'affording accurate quantitative information on instability in magnetrons and for affording quantitative informationen the useful energy content of radio frequency pulses generated by magnetrons.

A further object is to provide a pulsed magnetron instability detector circuit for providing quantitative information on malfunction of a magnetron which information is useful 4as a guide in the development of electronic and microwave circuitry utilizing magnetrons.

A further object is to provide a pulsed magnetron instability detector circuit for simplifying the establishment of test specifications for the stability of magnetrons andv for investigations of magnetron starting phenomena.

A further object is to provide a pulsed magnetron instability detector circuit for sampling the output microwave energy of a magnetron and for counting the number of pulses in the sample whose energy content is below a predetermined level.

A further object` is to provide a pulsed magnetron instability detector circuitv which is simple, does not rely on the use of particular electronic components or particular lay-out, does not require critical adjustments, and is suitable for operation in the vicinity of high-power pulse equipment.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the saine becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Fig. 1 is a block diagram of a preferred embodiment of this invention, and

Fig. 2 is a schematic wiring diagram in accordance with Fig. l.

Detailed information useful for designing a specimen of the invention shown in Figs. l and 2 for particular operating conditions, is available in a thesis entitled Pulsed Magnetron Instability Detector, submitted by the inventor in June 1953 in partial fulfillment of the requirements for the Degree of Master of Electrical Engineering at the Polytechnic Institute of Brooklyn in New York City.

The embodiment of the invention shown in block diagram form in Fig. l comprises two separate channels connected to a magnetron 10. The upper or reference channel consists of a delay .multivibrator 22. The delay multivibrator22 is adapted to be triggered by a sampling of leach negative voltage pulse generated by the pulser 12 for application to the cathode of the magnetron 10 under test. The delay multivibrator 22 generates a delayed positive pulse each time it is triggered. The lower channel includes a microwave filter 24 connected in conventional manner to the output end of the magnetron 10. Since frequencies of` spurious voltage modes of oscillation in properly designed magnetrons are separated from the normal or desired frequency by at least a filter of approximately 2% bandwidth blocks all frequencies corresponding to oscillation in undesired voltage modes. This bandwidth is sufficiently broad to a1- low for normal operating frequency changes without loss f signal from oscillations in the desired voltage mode. A crystal detector 26 is connected to the microwave lter 24. The crystal detector 26 receives from the microwave filter 24 the energy generated in the desired voltage mode of oscillation in the magnetron. Energy from other modes is blocked by the lter 24. An integrating circuit 28 is connected to the detector 26. The detected radio frequency envelope of energy generated in the desired voltage mode of oscillation only is coupled into the integrating circuit 28; the latter produces a voltage pulse for each magnetron pulse, whose peak amplitude is proportional to the total energy passing through the microwave lilter 24. The pulse produced by the integrating circuit28 is conventionally processed by an amplifier 32 connected to the output end of the integrating circuit 28, and cathode follower 34 connected to the output of the amplifier. The cathode follower presents a high impedancev to the amplifier and a low impedance source. A biased diode 36, normally nonconducting, is connected to cathode follower 34 and generates an output only if the peak amplitude of the integrated pulse exceeds the bias of the biased diode 36. A gate multivibrator 38 is connected to the output of the biased diode 36 and is triggered if the biased diode 36 conducts to a sufcient extent. The time delay introduced by multivibrator 22 is such that each gating pulse and the corresponding pulse from delay multivibrator 22 overlap. When the gate multivibrator 38 is triggered, it generates a gating pulse of sufficient pulse width and sutlicient amplitude for suppressing in a decoupling network 42, the corresponding pulse generated in the delay multivibrator 22. If the pulse from the delay multivibrator 22 is suppressedvthe' output stage 44 produces no output pulse for triggering the counter 46. However, if the gate multivibrator 38 is not triggered to generate a gate pulse for suppressing the pulse of the delay multivibrator 22 the latter' causes the output stage 44 to generate a pulse for triggering the counter 46. The counter 46 is triggered to register each pulse period of the magnetron during which instability reduces the useable radio frequency energy level generated during that pulse period below a predetermined minimum.

The description of the schematic Wiring diagram shown in Fig. 2 includes quantitative information on the components, immediately following the description of the components. It is not intended that the quantitative information be interpreted in a limiting sense since the stages may be varied to suit the magnetron operating conditions and equivalents of the stages shown be substituted in accordance with princi-ples well known in the art. The quantitative information supplied is for one particular design of this invention for operation where magnetron pulse widths are from 0.2 to 6.0 microseconds in duration and the pulse repetition rate exceeds 2000 pulses per second. The embodiment as described in detail Vhas beenvused satisfactorily and successfully under various test conditions within the range designated above. A'source'ofreference potential is indicated on the drawing by the conventional grounding symboland is referred to throughout this description as ground.- Where electronic tubes are shown on the drawings, the heater filaments have been omitted to simplify the drawings. The power supply, not shown, for the heater filaments, not shown, is conventional and is obtainable from electronics circuit designers handbooks.

The power supply serving as the source of plate supply voltage is not shown because a conventional power supply is suitable. For example, a single VR tube regulated volt power supply with an output of 35 milliamperes is ample for activating all stages of the dis closed circuit. information for designing a power supply rnay be obtained from many electronic circuit designers handbooks. Though the power supply is not shownon the drawing, the terminal to which it is to be connected is indicated 'by B+.

A fraction of the driving pulse for the magnetron 10 is coupled from the p ulser 12 to .a delay multivibrator 22 (Fig. l). The delay multivibrator 22 generates a fixed-width negative pulse (30 microseconds) which is differentiated to provide a'lpositive spike at the trailing end. The multivibrator 22 is a cathode-coupled oneshot multivibrator. It includes a normally conducting vacuum triode 52 (1/212AU7) and a normally nonconducting vacuum triode S4 (1/212AU7). The cathodes of both vacuum triodes SZfand 54 are commonly cor1 nected to a cathode bias resistor 56 Y(3 kilohms), the opposite end of the cathode bias resistor 56 being connected to ground. Plate load resistor 58 (20 kilohms) and 62 (20 kilohms) are connected between the source' of plate supply voltage B-land the plates of the vacuum triodes 52 and 54, respectively. A grid limiting resistor 64 (l megohm) is connected between the source of plate supply voltage B+ and the control grid of vac-- uum triode 52. Negative trigger pulses for the multi vibrator 22, having a fraction of the amplitude of the magnetron driving pulses, are derived from the pulser 12 (Fig. l) and are coupled into the junction between series-connected condenser 66 (.01 microfarad) and re sistor 68 (l kilohm), both of which are connected between the control grid of vacuum triode 52 and ground. A condenser 72 (.0025 microfarad) connects the plate of Vacuum triode 52 and the control grid of vacuum triode 54. A resistor 74 (220kilohms) connects the control grid of the vacuum triode 54 to ground. The long time constant of resistor 64 and condenser 66 prevents changes in the grid voltage of vacuum triode 52 during the interval during which it is cut off. In order for the grid circuit of the vacuum triode 52 not to have any effect upon the pulse-width output of delay multivibrator 22, `condenser 66 is caused to charge through a low impedance 68. If condenser 66 were to charge through the high impedance of the pulser 12 during the cutoff period of vacuum triode 52, such action would raise the grid potential of vacuum triode 52 by some fixed voltage dependent upon the ratio of the impedance of the pulser 12 to resistor 64 and would reduce the pulse width.

A differentiating circuit comprising a condenser 82 (l0 micromicrofarads) in series with a resistor 84 (500 kilohms), is connected between the plate of vacuum triode 54 and ground. Each time multivibrator 22 is triggered by a negative pulse (0.2-6 microseconds, 2- volts minimum) a corresponding negative pulse (30 microseconds, 70 volts) of constant pulse-width is generated at the plate of the normally nonconducting vacuum triode 54. The generated negative pulse, shown abovethegdelay multivibrator in Fig. 2, is differentiated to produce a negative'voltage spike across resistor 84' corresponding to the leading edge of the negative pulse and a positive voltage spike across resistor 84 corresponding to the trailing edge (both spikes having 0.2 microsecond pulse width, l35 volts amplitude). Only the positive voltage spike isl active to trigger the output circuit under those conditions when permitted to do so by the other o r radio frequency channel of the detector E .5 circuit. By feeding a narrow spike from. ditferentiator 82, 84 to the circuit 42 instead of the very much wider pulse from multivibrator ,52, 54, the likelihood of counter 46 being triggered by noise is minimized.

Microwave pulse energy generated by the magnetron is coupled into a conventional microwavefilter 24; filter 24 blocks all energy except that at the frequency corresponding to the desired voltage mode of oscillation in magnetron 10. Information on microwave filter design is obtainable from volume 9 of the Massachusetts Institute of Technology Radiation Laboratory Series by Ragan, published by McGraw-Hill. A properly designed magnetron generates microwave energy at a frequency corresponding to the desired pi mode and at frequencies corresponding to spurious modes of oscillation butrthe frequency of energy generated in the pi Inode differs by at least 5% from the frequency of the closest spurious voltage mode of oscillation. The microwave filter 24 is readily designed according to known principles for approximately 2% bandpass which is suitable for blocking microwave energy at all frequencies corresponding to all spurious modes of oscillation in the magnetron 10. The 2% bandpass is sufficiently broad to allow for normally operating frequency changes without loss of microwave energy corresponding the desired mode of oscillation.

A crystal detector 26 (IN23B) is connected to the output of the microwave filter 24. The crystal mount, not shown, is arranged to produce detected pulses of negative polarity. The input signal to the radio frequency channel is a sample or fraction of the pulsed microwave energy generated by the magnetron 10. Before these pulses can be integrated to analyze their energy content they must be detected to produce video pulses whose shapes are proportional to the envelopes of the modulated radio frequency. This detection is accomplished by means of the crystal detector 26. The crystal detector 26 is a microwave silicon crystal of the type employed as a mixer in the input to a superheterodyne receiver of a radar system.. A coaxial cable is used for connecting the crystal mount and the input end of the integrating circuit 28. Since normal coaxial cable lengths of 2 to 5 feet are short compared to the wavelengths of the highest frequencies composing the detected pulses, the coaxial cable capacitance indicated by 102 on the drawing is considered to be lumped (about 75 micromicrofarads) at the input to the integrator circuit. When the microwave energy is present, the equivalent capacity 104 of the equivalent circuit of crystal detector 26 and the shunt capacity 102 provided by the connecting coaxial cable both charge rapidly through the low forward resistance 106 of the crystal detector 26. However, at the termination of the last radio frequency cycle of the pulse the voltage of the equivalent capacity 104 of the crystal detector 26 causes resistor 108 to assume its high back value on the order of 10,000 ohms or higher. As a result, the crystal capacity 104 and the cable capacity 102 cannot discharge rapidly through resistor 108.

The integrator circuit 28 includes a voltage divider having a fixed resistor 112 (200 ohms) and a potentiometer 114 (100 ohms) connected in series and comprising a low load resistance to maintain the discharge timeconstant at a value suiciently low to prevent distortion of the pulse shape. The voltage developed across the cable capacity 102 is applied across the voltage divider. Potentiometer 114 provides for varying the input signal to the succeeding stages (eg. by 30%). A carbon potentiometer 114 and a carbon resistor 112 are used instead of wound elements because the signals are pulses as narrow as 0.2 microsecond and which have very high frequency components (5 megacycles). Any changes in the amplitude of a magnetron output microwave pulse due 4to mode instability is accompanied by a shift in frequency to a point outside the bandpass of the microwave filter 24 whereby the pulse does not reach the detecting crystal 26. Arcing and misiiring result in a complete absence of microwave energy output for the duration of the instability. Therefore, the microwave energy incident upon the crystal `is either at full value or zero. Therefore, it is not critical for the output voltage from the crystal detector 26 to bear a linear relationship with respect to input peak power in order to obtain a proper analysis of the energy content of the input signal. The crystal detector output voltage is either at full amplitude or at zero. The duration of the full amplitude pulse is a measure of the microwave energy generated in the desired Voltage mode of oscillation. There is no need for analyzing intermediate pulse amplitudes. The crystal operating region is a compromise (0.5 volt peak output obtained by incident power of about .2 milliwatt), i.e. the voltage is sufficiently low so as to be available from most microwave crystals yet is high enough to simplify problems encountered in shielding the circuit from external sources of noise, such as high power pulse generators and magnetron modulators, and a change in peak input power of 10% results in a 5% change in peak output voltage.

The integrator circuit 28 further includes a series of condensers 116 (690 micromicrofarads), 118 (1040 micromicrofarads), 122 (1610 micromicrofarads), 124 (2530 micromicrofarads), 126 (3910 micromicrofarads), 132 (9200 micromicrofarads), 134 (13,800 micromicrofarads); the condensers are connected between ground and the respective terminals of a multi-position selector switch 136 serving as a pulse-width switch. The contactor of the multi-position selector switch 136 is connected to one end of a limiting resistor 138 (l0 kilohms) connected at its other end to the tap of the potentiometer 114 and is Valso connected to the input end of amplifier 32; high value resistor 138 is provided to prevent loading of the detector by the integrating circuit. The range of sizes of condensers are provided in order that the integrator 28 be useful over a range of pulse widths (.02 to 6 microseconds) with approximately the same value of power incident upon the crystal detector. The condensers switched into the integrator cover the band in discrete ranges, each covering a 1.5 to l band in pulse width. The peak output voltage is small (about 0.02 volt). The amplifier 32 amplies the pulses produced by the integrating circuit (1500 to l). The width of the pulses before integration in the integrating circuit 28 vary depending upon the characteristics of the magnetron under test (0.2 to 6.0 microseconds). The amplifier 32 is a three-stage resistance-capacitance coupled triode amplifier. The first stage of the amplifier 32 includes a vacuum triode 152 (1/212AT7), a plate load resistor 154 (2 kilohms) connected directly between the plate of the vacuum triode 152 and the source of plate supply voltage B-land a cathode bias resistor 156 (50 ohms). A coupling condenser 158 (.001 microfarad) and grid-leak resistor 162 (l megohm), in series, are connected between the plate of the vacuum triode 152 and ground. The second stage of the amplifier 32 includes a vacuum triode 164 (1/212AT7), a plate-load resistor 166 (2 kilohms) connected between the plate of the vacuum triode 164 and the source of plate supply potential B+, and a cathode bias resistor 168 (50 ohms) connected between the cathode of the vacuum triode 164 and ground. The control grid of vacuum triode 164 is connected to the junction between the coupling condenser 158 and the grid-leak resistor 162. A coupling condenser 172 (.001 microfarad) and a grid-leak resistor 174 (1 megohm), in series, are connected between the plate of the vacuum triode 164 and ground. The third stage of the amplifier 32 includes a vacuum triode 176 (1/212AT7), a plate-load resistor 17S (10 kilohms) connected between the plate of the vacuum triode 176 and the source of plate supply potential B+ and a cathode bias resistor 182 (33 ohms) connected between the cathode of vacuum triode 176 'i and ground. Its control grid is connected to the junction between condenser 172 and resistor 174. A coupling condenser 184 (.001 microfarad) and a grid-leak resistor 186 (1 megohm), in series, are connected between the plate of the vacuum triode 176 and ground.

A cathode follower threshold circuit 34 is connected to the amplifier 32; it includes a vacuum triode 202 (1/212AT7) whose plate is connected directly to the source of plate supply potential B+, and a voltage dividing resistance connected between the cathode of the vacuum triode 202 and ground. The voltage dividing resistance comprises series-connected resistors 204 (667 ohms), 206 (476 ohms), 208 (357 ohms), 212 (278 ohms), 214 (222 ohms), and 216 (2 kilohms). Terminals of a multiposition threshold switch 222 are connected to the cathode and to the junction of each pair of adjacent resistors comprising the cathode resistance of cathode follower stage 34.

A biased diode 36 is connected to the contactor of threshold switch 222 and includes a crystal diode 232 (IN39)` and a voltage divider for biasing the diode 232 to a fixed threshold level. The gate multivibrator 3S is triggered if the threshold is exceeded. The voltage divider includes a pair of series-connected resistors 234 (70 kilohms) and 236 (5 kilohms) connected between the source of plate supply potential B+ and ground. A high-valued resistor 23S (l0 kilohms) is connected between the tap of the voltage divider and the cathode of diode 232. A bypass condenser 242 is connected across the resistor 236 of tthe voltage divider. IThe diode 232 conducts only when the potential at its anode end exceeds the bias voltage provided by the voltage divider.

A gate multivibrator is connected to the biased diode 36; it is a single-shot cathode-coupled multivibrator which when triggered generates a negative pulse of suficient amplitude (70 volts) and duration (60 microseconds) to suppress with certainty the positive reference pulse derived from differentiation of the output of the delay multivibrator 22. The gate multivibrator 3S includes a vacuum triode 252 (1/212AU7) and a vacuum triode 254 (1/212AU7). Plate load resistors 256 (100 kilohms) and 258 (4 kilohms) are connected between the plates of the vacuum triodes 252 and 254, respectively, and the source of plate supply potential B+. The cathode bias resistor 262 (1500 ohms) is connected between the commonly connected cathodes of the vacuum triodes 252 and 254 and ground. The plate of vacuum triode 252 is coupled to the control grid of the vacuum triode 254 by means of a condenser 264 (50 micromicrofarads). Grid-leak resistor 266 (900 kilohms) is connected between the control grid and cathode of vacuum triode 254. In the absence of a trigger pulse, vacuum triode 254 is operating at zero bias and is conducting because its grid is returned to its cathode through resistor 266. Vacuum triode 252 is cut off until there is a positive trigger pulse. A positive trigger pulse applied to the grid of vacuum triode 252 is required for triggering the multivibrator 3S.

The control grid of vacuum triode 252 is coupled to the biased diode by means of a coupling condenser 244 (.01 microfarad) in series with a resistor 245 (100 kilohms). A resistor 246 (500 kilohms) is connected between the junction of condenser 244 and resistor 245 and ground. YCondenser 244 and resistor 246 are designed for proper pulse width (60 microseconds) output.

When the diode 232 conducts, diode current serves to charge the coupling condenser 244 (0.01 microfarad), raising the potential on the control -grid of vacuum triode 252. The latter conducts and its plate voltage falls, thereby coupling a negative pulse to the grid of vacuum triode 254. Reduction of the plate current of this stage of vacuum triode 254 completes the regenerative feedback; this continues until vacuum triode 254 is cut oit and vacuum triode 252 is conducting. The capacitor 264 then discharges through resistor 266, resistor 262,

and conducting vacuum triode 252, until the voltage at the grid at vacuum triode 254 again is raised to the point where vacuum triodef254 can conduct and cause regeneration to establish quiescent conditions.

The threshold circuit' which includes the biased diode 36 and the cathode circuit of the cathode follower stage 34 permitsvan operator to preset the percentage of full pulse energy at the correct frequency that must be generated in order that the counter 46 not be triggered. The threshold circuit serves to determine the threshold quantity of signal energy generated in the desired voltage mode of oscillation and passing through the microwave filter 24, below which threshold quantity of signal energy the counter 46 is caused to register.

The decoupling network 42 is provided for combining the pulse generated by the delay multivibrator 22 and the gate multivibrator 38, if any; it functions as an anticoincidence device. The decoupling network 42 includes, in series, a crystal diode 272 (IN39) in series with a resistor 274 (200 kilohms) and a crystal diode 276 (IN39). A resistor 282 kilohms) is connected between the differentiating circuit at the output end of the delay multivibrator 22 and the decoupling network 42. A condenser 267 couples the plate of vacuum triode 252 to decoupling network 42. The decoupling network 42 combines pulses from the delay multivibrator 22 and the gate multivibrator 38 for coupling into output stage 44 without any interaction between the multivibrator stages. The diode 272 isolates the gate multivibrator 38 from the negative spike pulses corresponding to the leading edges of negative pulses produced by the delay multivibrator 22, while resistor 274 and diode 276 act as a clipper to limit the positive pulses. Germanium diodes are used in this grid network because they are compact and rugged, have low shunt capacities (approximately 1 micromicrofarad), have low forward resistance (285 ohms maximum), and do not require heat or power.

The output stage 44 comprises a vacuum triode 302 (1/212AU7) connected as a cathode follower and including a cathode bias resistor 304 (4 kilohms) connected between the source of plate supply potential B+ and ground. The cathode bias resistor 304' comprises part of a voltage divider for biasing the vacuum triode 302 beyond cutoff. The other part of the voltage divider is the resistor 306 (47 kilohms) connected between the source of plate supply potential B+ and the cathode end of cathode bias resistor 304. A bypass condenser 30S (500 micromicrofarads) is connected across the cathode bias resistor 304. The charging current of the gridcathode capacitance (1.6 micrornicrofarads) of vacuum triode 302 produces a spurious positive feedthrough (0.6 volt) when the tube is cut off. Since the spurious feedthrough is short in duration (0.5 microsecond) as compared to the normal signal (5 microseconds), the bypass condenser 308 connected across the cathode resistor 304 is used to bypass the spurious feedthrough and while it partially bypasses the normal signal output, it does not seriously aifect it. Through the use of the bypass condenser 308 the ratio of normal signal output to residual feedthrough is made very high. A coupling condenser 312 (0.02 microfarad) connected to the cathode of the vacuum triode 302 is used to couple the output developed in the cathode follower output stage 42 to a conventional electronic decade counter, not shown.

In operation, the missing radio frequency pulse detector circuit described when employed with a filter of approximately 2% bandpass, provides a positive output pulse to trigger an electronic decade counter each time a driving pulse is applied to the magnetron 10 under test, and the output energy within the bandpass of the filter is less than that of a normal pulse by some preset percentage. First the pulse-width switch 136 is set to the range that includes the pulse-width being employed for driving the magnetron 10. This selects the proper time constant for the integrating circuit 28. Then the asl-95,10*?

t9 threshold switch 222 is set to zero position and the .input levelpotentiometer 114 is set to the approximate center of its range. By means of a -variable microwave attenuator (not shown) in-the transmission line between the magnetron 10 and the crystal detector 26 the input signal is reduced 'below the threshold level so that the electronic counter records `thefull repetition rate of the trigger pulses. Then, by adjusting the input-level poten- .tiometer 114 as a fine control, the input to the radio frequency channelis adjusted tothe threshold at which the counter records the full repetition rate. Then the threshold switch 222 isset to :the desired operating position' corresponding'to the desired minimum-energy content of each radio frequency pulse. The counter `will then be triggered for all pulses -Whose energy content is less than that determined by the setting of the threshold switch 222. In other words, the counter -will register for all generated pulses Whose useful energy content .is less than that'of a normal pulse =by:a percentage selected through the use of switch 222. The missving radio frequency pulse detector circuit provides a quantitative information on magnetron malfunction. The quantitative information can be utilized as a guide in the development of electronic and microwave circuitry utilizing magnetron tubes. This circuit simplifes the establishment of test specifications for the stability of pulsed magnetrons, and aids materially in investigations of tube starting phenomena.

Obviously many modifications and variations of the present invention are possible inthe light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention mav be practiced otherwise than as specifically described.

.I claim:

1. An instability detector circuit adapted for coupling `to la magnetron and which is suitable for use in the vicinity of high power pulse equipment and its pulsing means, said detector circuit comprising; a first singlepulse generating means .adapted to :be coupled to the magnetron pulsing means for sampling a predetermined fraction of each of the magnetron driving ,pulses and for generating a pulse of predetermined pulse width for each magnetron drivingpulse; differentiating means coupled tovthe output end of said first single-pulse generating means to provide a narrow reference pulse coincident l y With the trailing edge of each pulse generated by said first single-pulse generating means, whereby the reference pulse is spaced by a predetermined interval from the corresponding driving pulse and whereby there is minimum likelihood of coincidence 'between` the narrow reference pulse and interference energy from said high power pulse equipment in the vicinity and consequently .minimum likelihood of modification of a significant number of the reference pulses by interference energy; a microwave filter adapted to be coupled to the output end of the magnetron for sampling apredetermined frac-tion of the microwave energy in a predetermined vfrequency band in each pulse |generated rby the magnetron; detector means coupled to said filter; integrating means coupled to said detector means for separately integrating the energy from said microwave filter for each magnetron pulse respectively; threshold means coupled to the output end of said integrating means; a second single-pulse generating means coupled to the output end of said threshold means and triggered each time said threshold means becomes conductive to generate a gating pulse of predetermined amplitude and Ymany times greater width than said reference pulse; said first single pulse `generating means delaying each reference pulse by an linterval such that each reference pulse occurs between the leading and trailing edges of a corresponding gating pulse; anticoincidence means coupled to the output ends of said differentiating means and said second single-pulse generating means for providing an output pulse for each reference pulse not attended by a gating rsampling a predetermined fraction `of-each of themagnetron driving `pulses and for generating one f`delayed reference pulse of very narrow pulse width for each magnetron driving pulse, whereby there =is minimum likelihood of coincidence between reference pulses and interference energy from said high Ipower pulse equipment in the vicinity and consequently yminimum likelihood of modification of a significantnumber of the reference pulses -by interference energy; second means ladapted .to be coupled to the output end of the magnetron forsampl-ing and summing upa predetermined fraction of the microwave energy in a predetermined frequency band in each pulse Agenerated by the magnetron and for. generating a -gating pulse of predetermined pulse width and much wider than the reference pulses for each magnetron driving pulse during which the microwave energy generated by the magnetron within-the selected frequency Iband exceeds a predetermined minimum; said first means delaying each reference pulse by `an amount such that theleading and trailing edges of each gatingpulse occur respectively before and after the corresponding reference pulse; third means coupled to the output ends of said first and second means for accepting the referencepu'lses and the gating pulses and for generating pulses whose number are related to .the numbercf reference pulses, that are attended `by gating pulses and the number of reference pulses that are not attended by gating pulses; and means coupled to the output end of said third means and responsive to-the pulses therefrom-for providing-an indication of the degree of-instability `in themagnetron.

3. A circuit adapted for detecting instability in a=mag netron and which is suitable for use in the vicinity of high power pulse equipment comprising: means for generating magnetron driving pulses adapted to lbe coupled to the magnetron; a first single-pulse generating means coupled to said means for sampling a ypredetermined fraction of each of the magnetron driving pulses and-for generating a pulse of predetermined pulse width for each magnetron driving pulse; differentiating means Ycoupled to the output end of said first single-pulse generating means to provide a narrow 'reference pulse coincident with the ltrailing edge of each pulse generated by said first single-pulse generatingmeans, whereby the reference pulse is `spaced -by a predetermined interval from the corresponding driving pulse-and whereby there is minimum likelihood of coincidence :between the narrow reference pulse and interference lenergy from said high power pulse equipment in the vicinity and consequently minimum likelihood of modica-tion of significant number of the reference pulses by interference energy; a microwave filter adapted to -be coupled to the output? end of the magnetron for sampling a predetermined fraction Vof the microwave energy in a predetermined 'frequency ba-nd in `each pulse generated by the magnetron; detector means coupled to said lfilter; integrating means coupled to said detector means for separately integrating the energy from sai'd mircowave `filter for each magnetron pulse respectively; threshold `means coupled Ato the output end of said integratingmeans; a second lsinglepulse generating means coupled Ito the output end fof -said threshold means and triggered each time said threshold means becomes conductive to generate a gating pulse of predetermined amplitude Yand many-times greater 'width than said reference pulse; said first single .pulse generating means delaying each reference pulse by an interval such that yeach referen'ce `-pulse voccurs between the leading and trailing edges of a corresponding gat- 'ing pulse; anticoincidence means Ycoupled to the output ends'of said differentiating means and said second singlepulse generating means for providing an output pulse for each reference pulse not attended 'by a` gating pulse; means coupled to the output end of said anticoincidence means to provide an indication of the degree of instability in the magnetron.

4. A circuit adapted for detecting instability in a magnetron and which is suitable for use in the vicinity of high power pulse equipment comprising; rst means for -generating magnetron driving pulses adapted to be coupled to the magnetron; second means coupled `to said iirst means for sampling a predetermined'fraction of each of the magnetron driving pulses and for generating one delayed reference pulse of very narrow pulse width for each magnetron driving pulse, whereby there is minimum likelihood of coincidence between reference pulses and interference energy from said high power pulse equipment -in the vicinity and consequently minimum likelihood of modification of a significant number of the reference pulses by interference energy; third means adapted to be coupled to the output end of the magrnetron for sampling and summing up a predetermined fraction of the microwave energy in a predetermined frequency band in each pulse generated by the magnetron and for generating a gating pulse of predetermined pulse width and much wider than the reference pulses for each magnetron driving pulse during which the microywave energy generated by the magnetron within the selected frequency `band exceeds a predetermined minimum; `saidtirst means delaying each reference pulse by `an amount such that the leading and trailing edges of gating pulse occur respectively before and after the corresponding reference pulse; fourth means coupled -to the output ends of said second and third means for acceptingl theV reference pulses and the gating pulses and for generating pulses whose number are related to the num- A'ber of reference pulses that are attended by gating pulses and the number of reference pulses that are Vnot attended by gating pulses; and means coupled to the output end of said fourth means and responsive to the pulses therefrom for providing an indication of `the degree of instability in the magnetron. Y

5. An antcoincidence circuit comprising: two diodes, a resistor connecting the cathode of one diode and the anode of the other diode, a second resistorV connected at -one end to `the anode of said one diode, a source of posi- "tive pulses coupled between the other end of said second resistor and the cathode of said other diode whereby curfrent flows through said resistors during each positive pulse therefrom, a source of negative pulses shunting said other diode, the negative pulses being of an amplitude such that when a positive and a negative pulse from said sources are coincident, the positive pulse is suppressed in said circuit.

6. An instability detector circuit adapted for coupling to a magnetron and its pulsing means, said detector cir- -cuit comprising; a first single-shot multivibrator adapted to be coupled to the magnetron pulsing means for samvpling a fraction of each of the magnetron driving pulses and for generating a pulse of predetermined pulse width 'for each magnetron driving pulse; dierentiating means coupled to the output end of said rst multivibrator to provide a reference pulse coincident With the trailing edge of each pulse generated by said rst multivibrator; a

microwave filter adaptedto be coupled to the output end of the magnetron-for sampling a predetermined frac- Vmeans to second multivibrator; the width of each pulse generated by said iirst multivibrator being such that Yevery gating pulse and the corresponding reference pulse overlap; voltage summation means including in series a rst unidirectional conducting means, a resistor, and a second unidirectional conducting means; a current limiting resistor connected between the output end of said diterentiating means and said rst unidirectional connecting means; a coupling condenser connected between the output end of said second multivibrator and the junction between said resistor and said second unidirectional conducting means of said voltage summation means; output amplier means connected at its input end across said voltage summation means and producing an output pulse only when a reference pulse from said differentiat- Aing means, not attended by a gating pulse from said second multivibrator, is applied to said voltage summation means, and a counter connected to said output amplifier means for counting pulses from said output amplier means to provide a quantitative measurement of instability in the magnetron.

7. A circuit adapted for detecting instability in a magnetron comprising; means for generating magnetron driving pulses adapted to -be coupled to the magnetron; a rst single-shot multivibrator coupled to said means for sampling a fraction of each of the magnetron driving pulses and for generating a pulse of predetermined pulse Width for each magnetron driving pulse; differentiating means coupled to the output end of said rst multivibrator to provide a reference pulse coincident with the trailing edge of each pulse generated by said iirst multivibrator; a microwave filter adapted to be coupled to the output end of 'the magnetron for sampling a predetermined fraction of the microwave energy in a predetermined frequency `band in each pulse generated by the magnetron; detector means coupled to said filter; an integrator circuit; adjustable voltage divider means coupling said integrating circuit to said detector means; means coupled to the output end of said integrator circuit for amplifying the output of said integrator circuit; a second singleshot multivibrator for generating a gating pulse of predetermined amplitude and pulse width when triggered; a biased diode coupling the output end of said amplitier means to second multivibrator; the Width of each pulse generated by said trst multivibrator being such that every gating pulse and the corresponding reference pulse overlap; voltage summation means including in ser-ies a first unidirectional conducting means, a resistor, and a second unidirectional conducting means; a current limiting resistor connected between lthe output end of said diterentiating means and said rst unidirectional connecting means; a coupling condenser connected between the output end of said second multivibrator and the junction between said resistor and said second unidirectional conducting means of said voltage summation means; output amplier means connected at its input end across said voltage summation means and producing an output pulse only when a reference pulse from said differentiating means, not attended by a gating pulse from said second multivibrator, Iis applied to said voltage summation means; and a counter connected to said output ampliiier means for counting pulses from said output amplier means to provide a quantitative measurement of instability in the magnetron.

References Cited in the rile of this patent UNITED STATES PATENTS 

